It occasionally is necessary to transmit both bias power and Pulse Width Modulation (PWM) duty cycle information across the isolation barrier, usually a transformer, of a circuit such as an isolated DC/DC converter, which receives an input DC voltage of particular magnitude and produces an output DC voltage of different magnitude. The output DC voltage may be of magnitude greater than, less than or equal to the magnitude of the input DC voltage, depending upon converter type. For example, in one type of isolated DC/DC converter implementing transformer isolation and pulse width modulation, termed a PWM DC—DC converter, information on one or more sequences of PWM duty cycle pulses corresponding to the desired output DC voltage magnitude, together with DC bias power, must be transferred from one side of the isolation barrier to the other side. This ordinarily is accomplished using a separate pulse transformer for coupling each PWM duty cycle sequence across the isolation barrier, and implementing sources of DC power on each side. It would be desirable to effect a means for combining transmission of bias power and PWM duty cycle information across the isolation barrier, in an efficient manner, so as to reduce component count and circuit complexity.
One known method for combining DC power and PWM duty cycle information is shown in FIG. 1, depicting a circuit 100 in which an isolation barrier between input and output is produced by a pulse transformer 102. At the primary side of the transformer 102 is a first DC blocking capacitor 104, and at the secondary side is a DC restorer circuit comprising a second capacitor 106 and diode 108, and a half-wave rectifier consisting of diode 110 and second DC filter capacitor 114. DC bias magnitude is limited by zener diode 112.
Although structurally simple, this circuit has several shortcomings. The value of voltage across the two DC blocking capacitors is determined by the PWM duty cycle information being transmitted across the isolation barrier. Duty cycle information can be lost completely for a period of time upon sudden and substantial change in the value of the duty cycle of the input waveform because the value of voltage across the two DC blocking capacitors cannot respond quickly enough. Another disadvantage of this type of circuit is that at low input PWM duty cycle, production of DC bias power is inadequate because the energy content of a low duty cycle waveform is small. And furthermore, a duty cycle of zero cannot be maintained for any substantial period without losing bias power transfer altogether. As another disadvantage of this type of circuit, there is no possibility of transferring more than one PWM duty cycle waveform simultaneously, as would be required in multiphase converter systems.
FIG. 2 depicts another type of circuit 200 commonly implemented to send PWM duty cycle information through a pulse transformer 202. In this Figure, the primary circuit of pulse transformer 202 comprises primary winding 202a in series with an RC circuit consisting of capacitor 204 and resistor 206, and the secondary circuit consists simply of a load resistor 208 connected in shunt with the secondary winding 202b of the transformer.
This circuit is at a disadvantage at least within the context of a PWM DC—DC converter in that it does not provide any means for transferring DC bias power across the transformer 202. In addition, if a turn-off pulse is missed, the effective PWM duty cycle transmitted across the transformer 28 can go to 100%, resulting in potential damage to the converter. Finally, because only a single PWM duty cycle waveform can be transmitted, multiphase operation is not possible.
The subject matter described hereinafter addresses and corrects the shortcomings of conventional technology, by providing circuitry and methodology for carrying out transfer of PWM duty cycle information and bias power across the isolation transformer of an isolated circuit, such as a PWM DC—DC converter, embedded in a constant repetition rate and PWM duty cycle carrier signal so as to avoid transformer saturation and optimize power transfer. The inventive circuitry and methodology described herein furthermore establish a prescribed maximum PWM duty cycle limit, such as 50% even if pulses are missed, allowing for transmission of two or more independent PWM duty cycle waveforms.